Question

A PMDC machine is measured to va120Vdc, ia 5.5A at the electrical terminals and the shaft is measured to have Tmech 5.5Nm, ns1200RPM. State whether the machine acting as a motor or a generator and state the efficiency of the system. Q11. Motor, 89.4 %(a) Generator, 89.4%(b) Motor, 95.5%(c) Generator, 95.5 %(d) none of the abov

Answer 1

(c) Generator, 95.5 %

Step-by-step explanation:

given data

voltage va = 120V

current Ia = - 5.5 A

Tmech = 5.5Nm

ns = 1200 RPM

solution

first we get here electric input power that is express as

electric input power = va × Ia ......1

put here value and we get

electric input power = 120 × -5-5

electric input power -660 W

here negative mean it generate power

and here

Pin ( mech) will be

Pin ( mech) = Tl × ω .........2

Pin ( mech) = 5.5 ×
`(2\pi N)/(60)`

Pin ( mech) = 5.5 ×
`(2\pi 1200)/(60)`

Pin ( mech) = 691.150 W

and

efficiency will be here as

efficiency =
`(660)/(691.150)`

efficiency = 95.5 %

so correct option is (c) Generator, 95.5 %